A mud motor 30 as shown in FIG. 1A is used in the oil drilling industry for directional drilling. The mud motor 30 is located on a bottom hole assembly 20 at the end of the drillstring 10 and has the drill bit 22 on its end. Inside a housing 32, the motor 30 has a rotor 40 and a stator 34.
During operation, drilling fluid F pumped downhole through the drill string 10 passes through the area between the rotor 40 and the stator 34. When urged by the pressures of the flowing drilling fluid F, the rotor 40 orbits inside the stator 34. A transmission shaft 24 inside the motor 30 converts the orbit of the rotor 40 to rotation of the drill bit 22, from which the drilling fluid F also exits. More rotational speed can be added to the drill bit 22 by simultaneously rotating the drill string 10 and the rotor 40.
The rotor 40 has a number of spiraling lobes 48 along its length that fit in complementary pockets of lobes 38 spiraling inside the stator 34 of the housing 30. For example, FIG. 1B illustrates an end sectional view of the motor housing 32 with the rotor 40 disposed inside the stator 34. Here, the rotor 40 has four lobes 48, and the stator 34 has five lobes 38.
To form the stator 34 for the motor 30, rubber is molded to the inside surface of the housing 32, which is typically a tubular or section of pipe. This process is outlined in U.S. Pat. No. 8,777,598. The molded rubber forms a multi-lobed cavity 36. The rotor 40, which is typically made of stainless steel, is inserted into the stator's cavity 36, and the rotor 40 generally has one less lobe than the stator 34. This allows the rotor 40 to orbit eccentrically in the stator 34 as the drilling fluid is pumped at high pressure in the cavities formed between the rotor 40 and the stator 34.
Over time, the rubber of the stator 34 becomes hard and brittle due to the high temperature environment downhole. This results in chunking and wear of the rubber seal of the stator 34 against the rotor 40. Eventually, drilling performance suffers due to wear and tear, and the motor 30 is retrieved from the hole. To rebuild the motor 30, the old rubber must be removed, and new rubber molded in.
Other downhole devices use rotors and stators, such as disclosed above. For example, a progressive cavity pump has a helical rotor and a rubber stator and is used to pump fluids in the cavities formed as the rotor is eccentrically rotated within the stator. These devices can also become worn overtime and may need rubber removal and replacement.
Removing the rubber stator material from a mud motor housing or the like has been done in various ways. For example, liquid nitrogen freezing has been used to remove the rubber from the motor housing 32. To do this, the whole housing 32 is submerged in liquid Nitrogen, causing the rubber to shrink and crack. Then, the rubber is pushed out of the housing 32 with a hydraulic ram. This process can be very expensive.
Burning is another technique that has been used to remove the rubber from the housing 32. Burning techniques are outlined in U.S. Pat. Nos. 2,291,862 and 6,966,105. To do this, the housing 32 is exposed to high temperature to effectively burn out, or break the bond of, the rubber to the stator 34. This creates noxious smoke and fumes, and the temperatures can damage the housing's integrity. The process is also slow.
A more popular form of removal uses high-pressure water blasting. The housing 32 is set up so that high-pressure water on the order of 20,000-psi to 40,000-psi at 200 Hp can be injected into the housing 32 at the rubber of the stator 34. The water blast is used to slice through the rubber and remove it in small chunks from the housing 32. Runoff water from this process must be cleaned of sediment and debris, requiring an expensive filtration system. Additionally, the process is slow and expensive.
Finally, it is conceivable that the rubber stator 34 can simply be milled, ground, or drilled out of the housing 32. As expected, the amount of debris from this would be considerable. Also, possible damage can occur to the inner surface of the housing 32 during the process, or an excess amount of material may still remain inside the housing 32, requiring further removal steps, such as water blasting.
Regardless of how rubber is removed, the inner surface of the cleaned housing 32 may again be molded with new rubber for a stator 34 so the motor housing 32 can be reused. During the rubber molding process to rebuild the stator 34, cores are used to form the inner profile. During the molding, excess rubber is typically added to the end of the housing 32 to ensure that the stator 34 is formed properly.
Once the housing 32 has been formed with the rubber stator 34 inside, the excess rubber inside the housing 32 towards the ends needs to be removed, bored to the housing's inner dimension, and chamfered with a 45-deg angle or the like. Traditionally, this is done using a large engine lathe. The motor housing 32 hangs out the end of the lathe as it is rotated, which complicates handling, and is unsafe. This process generally takes 30 minutes or more to complete for each end of the housing 32 and can be very cumbersome due to the exposed rotating housing.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.